1. Field of the Invention
The present invention relates to active optical cables (AOCs). More specifically, the present invention relates to methods for determining receiver coupling efficiency, link margin, and link topology in AOCs.
2. Description of the Related Art
An AOC is a fiber optic cable whose ends can be connected to at least one optical-to-electrical or electrical-to-optical converter, called an optical transducer. A fiber optic cable can have one or more strands of optical fibers. A full AOC includes a fiber optic cable with transducers on both ends, and a half AOC has a transducer on one end of a fiber optic cable, with the other end of the fiber optic cable connected to an optical connector. A full AOC can connect two electrical systems, e.g. two servers in a data center. A half AOC can connect an electrical system and an optical system. The end of the AOC contains the optical transducer so that the AOC can electrically transmit and receive data to and from the electrical systems while optically transmitting and/or receiving data through the fiber optic cable.
An AOC can either be uni-directional or bi-directional. A uni-directional AOC only transmits data in one direction, and a bi-directional AOC can transmit data in two directions. The AOC can contain a receiver that receives optical signals, a transmitter that transmits optical signals, or a transceiver that transmits and receives optical signals. A full uni-directional AOC includes a transmitter and a receiver. The transmitter receives electrical signals, converts the electrical signals into optical signals, and transmits the optical signals through the fiber optic cable to a receiver; and the receiver receives the optical signals from the fiber optic cable, converts the optical signals to electrical signals, and transmits the electrical signals. A full bi-directional AOC includes two transceivers to enable it to transmit and receive optical signals in two directions.
A full bi-directional AOC includes two transceivers to enable it to transmit and receive data in two directions. A full AOC is considered a closed link or system because the only optical signals transmitted by the fiber optic cables must be created by the two AOC ends that connect to the electrical connectors. A pair of half AOCs, either one transmitter and one receiver or two transceivers, can be mated together with an optical connector to form a closed link that can be opened. One reason to mate two half AOCs is to allow for increasing the length of the AOC by inserting an additional length of fiber optic cable.
In a receiver or in a receiving portion of a transceiver, the light exiting the fiber optic cable is directed to a photodetector. The photodetector has a known responsivity, which is typically expressed as an electrical current divided by the input optical power, i.e. A/W. The photodetector is connected to a transimpedance amplifier (TIA), which converts the current created by the light received by the photodetector to a voltage related to the amount of light incident on the photodetector. There are several types of TIAs, such as linear TIAs, limiting TIAs, and limiting TIAs with a received signal strength indicator (RSSI) output. For a linear TIA, the amount of light or optical power incident on the photodetector can be determined based on the linear TIA's known gain characteristic. The receiver coupling efficiency is the percentage of light exiting the optical fiber that the photodetector receives. Without a received signal strength indicator (RSSI) on a limiting TIA, the receiver coupling efficiency is difficult or impossible to measure. A focusing lens can be located between the end of the optical fiber and the photodetector. It is difficult or impossible to measure how well the optical fiber is aligned with the lens or how well the lens is aligned with the photodetector. This problem exists for both full and half AOCs.
In a transmitter or in a transmitting portion of a transceiver, electrical signals are converted into light by using a laser or some other light source, such as a light emitting diode (LED). Vertical-cavity surface-emitting lasers (VCSELs) can be used as the laser. The VCSELs can include an array of individually controlled lasers. The light from the laser is directed at the fiber optic cable. The transmitter coupling efficiency is the percentage of laser light entering the optical fiber. The optical fiber does not have a mechanism to indicate the amount of light that it receives. A lens can be located between the end of the optical fiber and the laser. It is difficult or impossible to measure how well the optical fiber is aligned with the lens or how well the lens is aligned with the laser for full AOCs. The optical coupling can be measured in a transmitter half AOC because the optical power that is coupled into the fiber can be detected using a commercial power meter.
Margin is the amount of loss a link can tolerate and still function properly. For example, if a transmitter puts out −1 dBm of power and if the receiver requires at least −10 dBm of power to function properly, then 9 dB of power loss between the transmitter and the receiver can be tolerated. Coupling efficiency and attenuation in the optical fiber will make up part of that 9 dB of power, and the rest is margin. The amount of margin cannot be measured in a closed link in which a full AOC or mated pair of AOCs are used because the receiver coupling efficiency can only be measured using half AOCs. Further, the receiver and transmitter coupling efficiency cannot be measured in a closed link of either a full AOC or a mated pair of half AOCs.
The power in the optical fiber and the power that reaches the photodetector are needed to determine the receiver coupling efficiency. Before mating a half AOC in a mated pair of AOCs, it is possible to measure the power in the optical fiber of the unmated half AOC. However, the receiver coupling efficiency cannot be determined because the power that actually reaches the photodetector cannot be determined.
The margin cannot be measured in full AOCs because it is a closed link with no additional connectors. There is an unknown transmitting coupling efficiency between the laser and the optical fiber on the transmitter. The amount of light that is in the optical fiber at the receiver is also unknown, which makes it impossible to know the receiver coupling efficiency between the fiber and the photodetector that is inside the receiver.
The margin cannot be measured in mated pairs of half AOCs, where there is an optical connector that mates the two half AOCs. To determine the amount of light that is coupled into the receiver, the optical power emitted by the laser, the transmission of the optical connection between the two half AOCs, and the coupling from the fiber after the connection in the receiver must be known. However, there is no currently known method to determine how much light actually reaches the receiver, which is the value that is of most concern.
The use of AOCs in systems can have many different optical fibers and connections placed between the two half AOCs. There is no known method to determine how much margin is in a link after the AOCs have been installed; the link simply functions or it does not.
It is possible to use eye-quality tests that can give a qualitative measurement of link margin, but eye-quality tests only provide a rough estimate of link margin. The bit-error rate can be measured by adjusting the optical power using an optical attenuator and measuring the frequency of errors as a function of the optical power. However, the bit-error-rate measurement is time consuming and can only be performed on half AOCs. Bit-error-rate measurement cannot be performed on full AOCs because bit-error rate measurement requires that the output power of the transmitter be adjusted to known levels while maintaining RF performance. While it is possible to change the drive current of the laser, which will adjust the transmitted power, it is difficult to change the drive current while transmitting a signal.
Some TIAs have an integrated RSSI function that measures the current emitted by the photodetector. However, not all TIAs have an integrated RSSI function. Without an integrated RSSI function in a limiting TIA, the photodetector current cannot be measured by the TIA. Because the current cannot be measured, it is not possible to determine how much light is being received by the photodetector.
In systems with numerous links, the topology of the links can be difficult to determine.